Tourmalinite-associated Pb-Zn-Ag mineralization at Bottino, Apuane Alps, Italy; geologic setting, mineral textures, and sulfide chemistry

Abstract

The Pb-Zn-Ag deposits of Bottino, Italy, are among the most significant in the Apuane Alps (NW Tuscany) mining district and were actively exploited in the past. The orebodies constitute a system of veins hosted within Paleozoic sedimentary and volcano-sedimentary sequences. These rocks were metamorphosed in the greenschist facies (350 degrees -450 degrees C, 3-4 kbars) during Tertiary Apenninic metamorphism (27-12 Ma). The orebodies are closely associated with tourmalinites and appear to be mostly conformable to the main wall-rock foliation (S 1 ) that developed during the first regional deformation (D 1 ). Locally, a relic pre-Apenninic foliation is recognizable both in the host rocks and in tourmalinites.The mineralogy of tourmalinites includes variable amounts of tourmaline, quartz, white mica, carbonates, chlorite, apatite, zircon, rutile, pyrrhotite, pyrite, arsenopyrite, and galena. Tourmalinites show a variable bulk-rock chemistry and are consistently enriched in Au, Ag, Sn, and W with respect to the host rocks. Tourmaline crystals show variable size and distribution, strong pleochroism, and irregular optical zoning. Their compositions belong to the schorldravite series and are similar to tourmalines from exhalative tourmalinites, whereas they differ significantly from granite-related tourmalines.The mineralogy of the ore veins is especially complex. Galena, sphalerite, pyrrhotite, arsenopyrite, pyrite, chalcopyrite, meneghinite, and Ag-rich tetrahedrite are the most widespread ore minerals. The occurrence of Ni minerals such as ullmannite and gersdorffite establishes a nickeliferous character for the Apuane Alps mining district, in contrast to other districts of the Tuscan metallogenic province.Textural features of the orebodies and ore minerals, and their inferred correlation with regional-scale deformational events, indicate that the mineralization at Bottino resulted from a complex, multistage process spanning at least the entire time of Apenninic metamorphism. Arsenopyrite geothermometry indicates an evolution of the ore-forming environment from temperatures close to peak metamorphic conditions ( approximately 450 degrees C), decreasing in a later stage to about 350 degrees C. The late appearance of monoclinic pyrrhotite replacing earlier hexagonal pyrrhotite indicates that temperatures eventually dropped to